Linear leads comprising conductors can couple with radio frequency (RF) fields, such as those used in magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). Examples of such leads include guidewires and/or interventional leads such as, for example, implantable pacemaker leads, spinal cord stimulator leads, deep brain stimulator leads, electrophysiology or other cardiac leads, leads used for implanted monitors, and leads used to administer a therapy during a surgical procedure. The coupling can sometimes result in local heating of tissue adjacent the lead(s) due to RF power deposition during the MRI/MRS procedure, potentially leading to undesired tissue damage.
MRI is a non-invasive imaging modality with excellent soft tissue contrast and functional imaging capabilities. However, MRI can be a contraindication for patients with implanted electrically conducting devices and wires, including cardiac pacemakers and/or defibrillators with leads connecting implantable pulse generators (IPGs), deep brain stimulation (DBS) electrodes, spinal cord stimulators, physiological monitors, etc . . . , for several reasons. For example, the electronics of the IPG/ICD may fail when in presence of the high magnetic fields, or the RF used in MRI may damage the circuitry of the IPG/ICD. In addition, the implanted lead may couple to local electric fields induced in the body during transmission of RF excitation pulses whereby the lead can unduly heat tissue adjacent the lead, or may propagate the RF to electrodes at the distal end of the lead or to the device or IPG to which it is connected, potentially causing local temperature rise to unsafe levels and/or damage to the implanted device. The heating problem has been reported in the scientific literature by researchers.
For example, Luechinger et al. reported a local temperature rise of 20° C. in tissue adjacent to pacemaker leads implanted in pigs during an MRI scan. See, Luechinger et al. In vivo heating of pacemaker leads during magnetic resonance imaging, Eur Heart J 2005; 26(4):376-383. In addition, Rezai et al. reported in vitro tissue heating in excess of 20° C. adjacent to DBS (deep brain stimulation) leads during an MRI scan. Rezai et al., Is magnetic resonance imaging safe for patients with neurostimulation systems used for deep brain stimulation? Neurosurgery 2005; 57(5):1056-1062. Even external leads such as those used for measuring and monitoring physiological signals (electrocardiograms, EKG, electroencephalograms, blood pressure, sonography) during MRI may be subject to heating.
One approach to allow patients with implanted devices, such as IPGs and leads to be scanned by MRI, is the use of strictly controlled conditions that limits the input power of the MRI RF pulse sequences. This approach is reported by Gimbel et al., strategies for the safe magnetic resonance imaging of pacemaker-dependent patients, Pacing Clin Electrophysiol 2005; 28(10):1041-1046, and Roguin et al., Modern pacemaker and implantable cardioverter/defibrillator systems can be magnetic resonance imaging safe: in vitro and in vivo assessment of safety and function at 1.5 T. Circulation 2004; 110(5):475-482.
In other (non-MRI) uses of RF, such as where external RF electromagnetic (EM) energy is present and/or used for therapeutic purposes, external or implanted leads may also couple to the applied RF EM field and cause unsafe tissue heating or damage or destroy electronic devices that can be connected thereto. For example, RF diathermy or ablation or cauterization of tissue can sometimes employ implanted or intra-body leads that may also couple to the applied RF EM field and cause unsafe tissue heating, such as that reported for a patient undergoing RF diathermy. See, Nutt et al., DBS and diathermy induces severe CNS damage, Neurology 2001; 56:11384-1386; and Ruggera et al., In Vitro assessment of tissue heating near metallic medical implants by exposure to pulsed radio frequency diathermy, Physics in Medicine and Biology, 48 (2003) 2919-2928. Another non-MRI example of where such EM-field coupling may occur is where individuals with implanted leads are in close proximity to EM field transmitters such as RADAR, TV, wireless telephone, radio facilities, fixed or mobile. Similarly, EM-coupling may also occur with external-conducting leads connecting electronic equipment that are sensitive to intense EM fields close to intense EM field sources.